Linear Technology Analog Devices LT8301 Datasheet page 17

APPLICATIONS INFORMATION
Step 4: Choose the Output Capacitor.
The output capacitor should be chosen to minimize the
output voltage ripple while considering the increase in size
and cost of a larger capacitor. Use the equation below to
calculate the output capacitance:
2
L •I
PRI SW
C
=
OUT
2 • V • ΔV
OUT OUT
Example:
Design for output voltage ripple less than 1% of V
i.e., 50mV.
40µH • (0.86A)
C
=
OUT
2 • 5V • 0.05V
Remember ceramic capacitors lose capacitance with
applied voltage. The capacitance can drop to 40% of
quoted capacitance at the maximum voltage rating. So a
100µF, 10V rating ceramic capacitor is chosen.
Step 5: Design Snubber Circuit.
The snubber circuit protects the power switch from leak-
age inductance voltage spike. A DZ snubber is recom-
mended for this application because of lower leakage
inductance and larger voltage margin. The Zener and the
diode need to be selected.
The maximum Zener breakdown voltage is set according
to the maximum V :
IN
V ≤ 65V – V
ZENER(MAX)
Example:
V ≤ 65V – 32V = 33V
ZENER(MAX)
2
= 60µF
IN(MAX)
For more information
A 20V Zener with a maximum of 21V will provide optimal
protection and minimize power loss. So a 20V, 0.25W
Zener from Central Semiconductor (CMDZ5250B) is
chosen.
Choose a diode that is fast and has sufficient reverse volt-
age breakdown:
V > V
REVERSE SW(MAX)
V = V
SW(MAX) IN(MAX)
Example:
,
OUT
V > 53V
REVERSE
A 100V, 0.25A diode from Central Semiconductor
(CMHD4448) is chosen.
Step 6: Select the R
Use the following equation to calculate the starting value
for R :
FB
N • (V
PS
R
=
FB
100µA
Example:
3 • (5V + 0.3V)
R
=
FB
100µA
Depending on the tolerance of standard resistor values,
the precise resistor value may not exist. For 1% standard
values, a 158k resistor should be close enough. As dis-
cussed in the Application Information section, the final
R value should be adjusted on the measured output
FB
voltage.
www.analog.com
+ V
ZENER(MAX)
Resistor.
FB
+ V )
F
OUT
= 159k
LT8301
Rev. A
17